<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        div {
            width: 300px;
            background-color: rgba(0,0,0,0.3);
            padding: 10px; /*设置四个方向的内边距,实现缩进效果*/
            position: relative; /*设置相对定位,身为上级作为img的参照物*/
        }
        input {
            width: 260px;
            border: 0;
            padding: 10px 20px;/*内边距影响宽高,所以将300修改为260*/
        }
        img {
            position: absolute; /*绝对定位*/
            right: 40px;
            top: 14px;
        }
        p {
            font-size: 12px;
            color: red;
            margin: 0;
        }
    </style>
</head>
<body>
    <div>
        <input type="text" placeholder="请输入用户名">
        <img src="http://doc.canglaoshi.org/tstore_v1/images/login/yhm.png" alt="">
        <p>用户名不能为空!</p>
    </div>
</body>
</html>